CARAJÁS NATIONAL FOREST, Brazil — Archaeologists must climb tiers of orchid-encrusted rain forest, where jaguars roam and anacondas slither, to arrive at one of the Amazon’s most stunning sights: a series of caves and rock shelters guarding the secrets of human beings who lived here more than 8,000 years ago.

Almost anywhere else, these caves would be preserved as an invaluable source of knowledge into prehistoric human history. But not in this remote corner of the Amazon, where Vale, the Brazilian mining giant, is pushing forward with the expansion of one of the world’s largest iron-ore mining complexes, a project that will destroy dozens of the caves treasured by scholars.